硅异质结太阳电池的物理机制和优化设计

硅异质结太阳电池是一种由非晶硅薄膜层沉积于晶硅吸收层构成的高效低成本的光伏器件,是一种具有大面积规模化生产潜力的光伏产品.异质结界面钝化品质、发射极的掺杂浓度和厚度以及透明导电层的功函数是影响硅异质结太阳电池性能的主要因素.针对这些影响因素已经有大量的研究工作在全世界范围内展开,并且有诸多研究小组提出了器件效率限制因素背后的物理机制.洞悉物理机制可为今后优化设计高性能的器件提供准则.因此及时总结硅异质结太阳电池的物理机制和优化设计非常必要.本文主要讨论了晶硅表面钝化、发射极掺杂层和透明导电层之间的功函数失配以及由此形成的肖特基势垒;讨论了屏蔽由功函数失配引起的能带弯曲所需的特征长度,即屏蔽长度;介绍了硅异质结太阳电池优化设计的数值模拟和实践;总结了硅异质结太阳电池的研究现状和发展前景.

Physical mechanism and optimal design of silicon heterojunction solar cells

Silicon heterojunction (SHJ) solar cells are crystalline silicon wafer-based photovoltaic devices fabricated with thin-film deposition technology. The SHJ solar cells hold great potential for large-scale deployment for high conversion efficiencies with low-cost manufacturing. Recently Kaneka Corporation has fabricated an interdigitated-back-contact (IBC) SHJ solar cell with a certified 26.33% conversion efficiency in a large area (180.4 cm²), which is a world record for any 1-sun crystalline silicon wafer-based solar cell. The key feature of SHJ solar cells is the impressive highopen-circuit voltages (V_oc) achieved by the excellent amorphous/crystalline silicon interface passivation. Generally, in SHJ solar cells, the boron doped hydrogenated amorphous silicon (p)a-Si:H] serves as hole collector and the phosphorus doped hydrogenated amorphous silicon (n) a-Si:H] functions as electron collector. In order to improve the lateral carrier transport of these layers, transparent conductive oxides (TCOs) are usually deposited on both sides of the solar cell. Therefore the parameters such as the heterointerface passivation quality, doping concentration and thickness of the a-Si:H doped layer, and work function of the transparent conductive oxide layer are the key factors that affect the performances of SHJ solar cells. Enormous research efforts have been devoted to studying the effects of the aforementioned influencing parameters on the photovoltaic characteristics of SHJ solar cells. Some research groups have addressed the physical mechanism behind the limitation of the solar cell efficiency. Owing to the insight into the physical mechanism some guidelines for optimally designing the high-performance solar cells in future are obtained. It seems therefore important to summarize the research efforts devoted to the physical mechanism and optimal design of SHJ solar cells.In the present review, we mainly discuss three important issues: 1) the amorphous/crystalline silicon interface passivation; 2) the Schottky barrier resulting from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer; 3) the screening length that is required to efficiently shield the parasitic opposing band from bending originating from the work function mismatch between the (p)a-Si:H doped layer and the transparent conductive oxide layer. The numerical simulation and optimal design of SHJ solar cells are analyzed, and three strategies that may improve the solar cell performances are presented: 1) a hybrid SHJ solar cell structure with a rear heterojunction emitter and a phosphorus-diffused homojunction front surface field; 2) replacing the (p)a-Si:H doped layer by higher doping efficiency microcrystalline silicon alloys such as μc-Si:H, μc-SiO_x:H or μc-SiC_x:H; 3) replacing the (p)a-Si:H doped layer by higher work function transition metal oxides such as MoO_x, WO_x or VO_x. Finally, the research progress and future development of SHJ solar cells are also described.